Good point and I don't know if the "motor" aspect of a motor home can ever be made efficient with current technology and certainly not with 1971 technology like this truck, but we can look at the design of the "home".

If there's one thing certain about motor homes it's that they can be very uncomfortable in extreme temperatures, heat is the worst offender for two reasons, it is more difficult to get away from and, in general, motor homes are used to migrate to warmer climates. Therefore the emphasis of this essay is on controlling heat but cold is a problem as well so it won't be ignored.

Motor homes generally cannot provide a lot of electrical energy because of size limitations. Even if yours is large enough to be able to generate the required energy it's very expensive to do so and many motor homers are full timing on strict budgets. Therefore the purpose of this essay is to discuss some methods of increasing the comfort level of a motor home in both hot and cold climates without throwing masses of energy and money into heating and cooling.

I have spent a lot of time looking into solar house design. I even designed a solar house once then realised how expensive it is to build anything different in Australia and backed out before it cost me too much. In Canberra at least, brick veneer is the norm, anything else attracts a premium. These days I'd build it myself, but that's another story.

Anyway the experience should help in my motor home design. The difference being of course that a house stays in one place, sunlight angles etc can easily be calculated. A motor home can be in Darwin this month and Hobart the next, it can be parked north-south today and east-west tomorrow. It seems reasonable then to assume that the energy efficiency of a motor home cannot equal that of a house. Still it makes sense to try and be as efficient as possible.

One problem southerners have when designing a motor home is that they think about preparing the vehicle for cold weather. While cold shouldn't be ignored (it can get VERY cold at night in central Australia) it's usually the purpose of a motor home is to spend as much time as possible in warmer areas, so protecting against heat should be the main design factor. If you live in a cold climate you have to change your mind set to think hot. Fortunately I used to live in the tropics so this shouldn't be too hard to do. 

By and large I do not want to use air conditioning in the motor home, we do have one but to date have only turned it on once to see if it works. My plan is not to drive to Australia's classic spots then sit inside an air conditioned box watching TV.

However I know it can get very uncomfortable in the heat and humidity and air conditioning solves both of those problems by cooling and drying the air.

Another problem with relying on air conditioning is that you can't really use it when bush camping. The electrical current required is far too much for your batteries to provide for very long so you must run the genset. This is usually noisy and always expensive. Remember that the genset that you thought was nice and quiet in the suburban backyard can be a real pain in the silence of the outback.

If you spend all/most of your time hooked up to power in caravan parks then it's a different story. Why not use the aircon, you're paying a fortune for the power anyway.

However, most people reading this are interested in bush camping so I say, forget the airconditioner unless there's a likelihood that you'll be spending a lot of time time in caravan parks (say you get a job in Darwin).

What will keep you cooler is airflow. Ventilation is obviously the cheapest and most energy efficient method of getting a breeze through the motor home and your design should be such that it traps any breezes. But there isn't always a breeze to trap.

Therefore, at times, you will need to generate your own breeze with fans. I think you need two fans in the lounge rather than a single oscillating fan. I say this because I don't find the oscillating fans very efficient in their usage. If you can limit the oscillation (not possible with all models) and sit right next to each other (assuming there's two of you) then very little is wasted. However if you have separate chairs spaced a metre or so apart then the fan spends a lot of time directing air to the gap between you.

We use two normal household desk fans but installed them horizontally so we have personal ceiling fans. This works well.

It can be very hard to get to sleep when it's hot so I also think a couple of fans in the bedroom are the go. Once you're asleep they are probably not needed so controlling these fans with timers is a good idea.

There is however no substitute for passive cooling in the form of good ventilation so let's look at that.

I believe we should be trying to be comfortable using passive techniques as much as practical, and this means plenty of ventilation. Most modern motor homes have windows that are tiny for the size of the rig. I've even seen motor homes with windows that don't even open; I find this to be incredible.

I reckon you need very large windows, the larger the better and we have windows about 1.2 x 2.7 metres in size in the lounge room. Unfortunately commercially available windows are not made this large or have what I consider to be serious limitations, for example,

• The normal wind-out caravan-style window cannot be made large enough because of the weight of the glass or perspex,
• The large windows are usually made up of several small sections, to my mind this breaks up the view through the window too much,
• Another way to make large windows is to have them open by sliding. This only allows the window to be half open at best.

All in all I could not find an adequate solution commercially so I decided not to have any glazing at all for most of the "windows". Most of the openings in our motorhome don't have any glass, just fly screens.

It's not good enough just to have large opening windows, you must also think about their placement. Ideally you want any airflow to be directed through as much of the motor home as possible. If you have two windows in the rear corner then that's where you'll have to sit to benefit from the breeze.

Also, many motorhomes have windows in the bedroom but they are above the bed. This is ludicrous, the windows should be on opposite sides of, and at the same level as, the bed. What's the point in a breeze wafting a foot or so above you?

Shutters
We find that not having any glazing works well but then our rig has large shutters over each window. If the weather is cold or windy we close some or all of the shutters. In this situation a large skylight still provides enough light to live by.

The main shutters can be opened to different amounts so, for example, we can close the windward-side shutter and half open the leeward-side shutter on a hot but rainy day.

More about shutter in the "shade" section below.

Roof vents
Because the hottest air in the motor home will be up at the ceiling a roof vent can be an efficient extractor of this air. Roof vents can also double as skylights but remember that letting light in is pretty much the same as letting heat in.

Rain
Whatever ventilation you implement, be sure that it can be used in the rain. Just because it's raining doesn't mean that it's cold, even in the southern states. I once spent a very uncomfortable weekend in my 4x4, even though it was bucketing down it was still quite warm and humid yet I had to have the vehicle closed up to keep from getting wet. And this was in May in southern NSW. In the tropics it's even worse.

Some roof vents can be used in the open position in the rain without letting water in. Some open on four sides so you can just close one or two sides according to the direction of the rain. As for windows, I have to admit that the wind-out style is best here as they can usually be left fully open even in heavy rain. As I mentioned we have shutters protecting the windows so we are able to have the windows open even when it's raining.

The insulation was doing exactly what it was designed to do, slow the transfer of conducted heat. But it's a two-edged sword.

Therefore, in an environment that is cycling between hot and cold, in the desert for example, insulation can be very effective as long as you have a method of cooling the interior in the evening.

However in an environment that is constantly hot, say in the tropics, insulation will be of little use in protecting you from the constantly high ambient temperature.

The above assumes that you do not want to use active methods to heat or cool the motor home. If you are happy to use an air conditioner then it's a different story and of course you'll be using a heater occasionally. When you are actively pumping heat or cold into the motor home's interior and don't want it escaping to the outside, the more insulation the better.

So the bottom line is this, given that you will be living in a variety of climates it makes sense to insulate as well as you can. In most of these climates the insulation will be of benefit but in a constantly hot climate its usefulness is debatable.

Often people add masses of insulation to the majority of the walls only to forget the areas that are directly piping heat into the motor home's interior.

With a metal skin on a metal frame the motor home body is a very efficient heat transfer device. And this transfer is directly into your living space. There's little point insulating the gaps between the frame if heat is entering through the spaces between the insulation, ie the frame itself.

I found that painting the interior of the body with an insulating paint stopped a lot of the heat transfer and this is something that anyone building a motorhome, or converting a bus, can do.

The design of busses, with all those windows, does not lend itself to thermal isolation of the inside walls. If possible, remove unwanted windows and sheet as much of the body as possible. Any remaining windows should have awnings or shutters to stop the heat entering.

When designing your motor home always try to think of ways that heat can enter the structure.

This is a case of "do what I say, not what I do" because my motor home is actually all natural aluminium. This is very reflective and if you walk passed out motor home on the sunny side you'll get heat stroke from the reflection. This is all heat that is not entering the vehicle.

The roof is probably the most important part, as it will take the brunt of the sun's rays, try to at least paint the roof white.

I was just about to write off thermal mass as a technique for motor homes when I remembered something I was going to try years ago. Water is a very good thermal storage medium. What about using a heat exchanger on the roof, another inside the motor home, and your washing water tank as the storage device? In winter (or in the desert) heat could be stored during the day by circulating the water through the roof exchanger, then used at night buy circulating through the exchanger in the living area. In summer you circulate externally at night to cool the storage then circulate internally during the day. Small circulating pumps, as used by solar hot water systems, use very little power, as this is a closed system and therefore balanced.

Just a though, I might spend a bit of time looking into this.

Tropical roof
Those who have been up north will have seen many vehicles with a second skin to the roof. This is called a "tropical roof" and is usually placed about an inch or so above the actual roof and open on all sides to allow airflow. A tropical roof is effectively a way to carry your own shade wherever you go. It will take the brunt of the sun's radiation and for best results should be thermally isolated from the real roof.

As the tropical roof heats up it will heat the air beneath it. This will cause convection currents that will replace the hot air with cooler air from the gap around the roof. This is why at least two opposite sides of the tropical roof need to be open. If the roof is sealed the air sandwiched between the two roofs will simply heat up and transfer the heat to the real roof.

It's also very common up north to see an aluminium boat (a "tinny") carried, upside down, on the roof of vehicles. Apart from being a convenient way to carry the boat this is also doubling as a tropical roof.

Naturally if your solar panels are mounted on top of the rig with an air gap between them and the roof then they perform this function, at least for the area they cover.

The problem with windows and curtains
As mentioned above, another weak point in the thermal design of motor homes is the windows, especially with busses. Let's get one thing straight, you can put curtains or blinds up 'till you're blue in the face but, once the light (read heat) passes through a window the damage is done.

Why is this?

The sun's radiation has a short wavelength that can easily pass through glass. When this radiation reflects off an object inside the motor home the wavelength lengthens to a point where the radiation can no longer pass through glass. Therefore it is trapped. This is why greenhouses work so well.

Curtains do help but the radiation is still inside and, as energy cannot be destroyed, it must be converted. This conversion takes the form of heating the air between the curtains and the window, this hot air rises through the top of the window frame and sucks cooler air from the bottom. This is a convection current and it will quickly heat your motor home.

There is a way to minimise this problem; use pelmets and insulated curtains. If the top of the window frame is closed the hot air cannot escape into the motor home's interior. Convection currents will still occur but they will be confined to the space between the curtain and the window. This area will of course get very hot and you will still get a lot of heat transferred by conduction through the curtains. If your curtains are made of insulating material this will be minimised as well.

Shutters
The best way to stop the build the up of heat is to stop it entering the motor home in the first place by placing the windows in shade. Possibly the best way to do this is with awnings and I'll cover them in the next section. However I have used shutters in my design for the following reasons (note that I'm talking about large one-piece shutters similar to an awning, not the slatted venetian blind style),

• They provide shade in much the same way as awnings.
• My vehicle is an off-road motor home so it will spend a lot of time in close contact with trees, therefore my windows need protection.
• Shutters will provide an extra seal against the fine bull dust.
• The air gap created when the shutters are closed will help insulate the motor home.

Awnings
Unless someone drops a thermo nuclear device within a few miles of your motor home the largest radiant heat source of all is the sun, however there is another contender. Reflected and radiated heat from the ground. Reflected heat is simply the sun's rays reflecting from the surface. However not all the sun's energy is reflected, some is converted to heat in the ground and this in turn is radiated. I'm sure you appreciate that it's cooler on grass than concrete and that's why.

The most common awning used on motor homes is the rollout style as made by Fiamma and Electrolux et al. In general these do a good job. However, on an off-road motor home there are not very practical because they protrude from the vehicle's side and will easily be torn off by passing trees. It's possible to inset the awning and, in an off-road motorhome that makes sense.

As mentioned before the best way to stop heat entering the motor home is to stop it from reaching it in the first place. This is where awnings come into their own. A large awning stretching for the majority of the motor home's length will very effectively shade the vehicle but it will also shade the ground beside it. This will reduce both reflected and radiated heat from the ground.

Most rigs have the awning on the left-hand side but, if you can afford it, put a full-length awning on both sides of the rig, obviously this will be good for heat reduction but it has an added benefit. You can't always park with the pleasant view on the left side so having awnings on both sides gives you some flexibility. Also, when it's hot you can swap sides during the day and sit on the side away from the sun. I've seen a few rigs with awnings on both sides and it seems to work well.

Using the rule of thumb of five hours useful sunlight per day, 2500 watt hours in five hours means an instantaneous charging rate of 500w or 40 amps (at 12v) or about eight 64w panels. In the summer and/or northern areas you would get more that this, in Tasmania be prepared to fire up the genset or cut back on usage. Therefore with a large rig I believe eight 64w panels is about right. Many people get by with a lot less, you have to look at your own usage.

Now, how do we mount them?

Most people will mount the panels on the roof and the simplest way is to just bolt on some brackets to the roof then screw the panels to the brackets. Make sure there is an air gap to help cool the panels as they lose efficiency when they get hot (not true for the Unisol panels, they actually increase their output a little in the heat).

I've seen a motor home with a massive row of panels mounted along the side or the rig. When the owners set up camp the panels were lifted out from the bottom and tilted to better catch the sun's rays. This had a beneficial side affect of providing shade for the side of the rig. If you do this make sure that the panels can lift to the horizontal position otherwise you may be forced to park in a specific direction, ie east west.

Tracking the sun
Many people ask if it's worth the effort to allow the panels to be tilted so they can be angled to directly face the sun and therefore more efficiently catch the rays. As I understand it, a system that automatically follows the sun will generate about 10% more electricity. But this is at the expense of building and installing a complicated system that has to tilt in two planes because the panels have to be laid down while travelling and you are living at various latitudes. If you have to move the panels manually then you will forget half the time or be away from the rig and unable to do so. In either case the complicated mounting is wasted.

One other thing to consider is this. As I've mentioned before, motor homes are generally used in the summer and/or northern areas. In both these cases the sun is very high in the sky so the panels would be laid almost flat in one plane anyway but have to be lifted from the roof to allow tilting in the other plane.

Also in these conditions there will be more than the standard five hours useful sunlight per day. One might think this is an argument for building a simpler mount that only allows tracking in one plane but if you do this you will have to park east west. Far too big an inconvenience if you ask me.

Also, remember that all this extra mounting will be far more prone to failure than a simple bracket. In general it is considered not worth the effort to track the sun with a motor home mounted solar panel array.

Removable panels
What about having removable panels so they can be placed in the sun while the rig's in the shade? This is a common technique with four wheel drivers and motor homer's who only have one or two panels. It's not a bad idea and in many ways gives you the best of both worlds, ie sun for power and shade for comfort. The panels are easily stolen however and placing any more than one or two would quickly become a real chore.

Broad spectrum panels
Part of the problem with parking in the shade can be overcome by using the Unisol broad-spectrum panels. In normal sunlight these panels are less efficient per unit area than most panels, but they respond to a greater range of wavelengths. What this means is that, for a given wattage, they are slightly larger than the normal panel but work much better in the shade and on overcast days. I've heard stories from people with Unisol panels who have been charging their batteries while parked under a street light.

As I said these panels are slightly larger, the average 64w panel is about 600x1200mm, Unisol panels are roughly 700x1400mm for the same wattage. This may, or may not, be an issue in your design.

Another feature of the Unisol panels is the fact that they have no glass. The cells are encapsulated in a resin so the panels are very tough and should have no problem with hail or tree branches.

Yet another feature of the Unisol panels is the fact that each row of cells is diode isolated from the others. Most solar panels will totally fail to produce current if only one cell is shaded. As the cells are about 4" in diameter it's easy to loose the entire panel output with just a little bit of shade. Shading one cell of a Unisol panel only looses the output from that row of cells, not the entire panel.

"For Sale. 20' motor home, all mod cons, island bed at rear". I used to read ads like this and wonder why the island bed was featured so prominently. In my mind it was a massive waste of space to devote up to half of the floor plan to the bedroom. I understood that it would be easier to make the bed with such a design but that's not enough reason to lose all that living space and certainly not something to feature in an ad.

Then I realised why it can be such a selling point to some people. Older motor homers don't want to climb ladders to get into bed, some probably can't even do so. Combine this with the knowledge that most older people have wizened up to the fact that you don't need many material possessions to enjoy life (and therefore don't need as much storage space), and suddenly an island bed is important.

Never the less, I'm only forty something, I still need my toys and I'm happy to climb ladders. Therefore our bedroom is over the cab.

Most cab over designs have the bed above the cab in a small claustrophobic, often totally unventilated, cubby hole. This is nice and cozy on a cold night in Kosiusczko but unbearable on a December evening in Kakadu. Very early in the design process, and after seeing a couple of examples at CMCA rallies, I decided that a pop top design was the answer however there were problems with designs I saw, namely,

• Often the top had to be raised to use the bedroom. I though this was very inconvenient for a quick roadside sleep and also draws attention to the fact that you are camping.
• If the sides are made of fabric and insect screen we would have good ventilation but poor insulation.

Let's look at problem number one. Having to raise the top even for a quick snooze in a rest area is a pain and a real design limitation. Also, as most motor homers know, sleeping in rest areas is often frowned upon by the authorities so by not raising the top you don't attract attention to the fact that you are camping. But there is another reason, what's the definition of camping? In a recent Wanderer a member pointed out that, at least with one local ordinance, camping involved "erecting a structure for the purposes of sleeping" or some such. A raised pop top would qualify as an erected structure but with no raised pop-top there's no "erected" structure.

I'm splitting hairs I know and I wouldn't like to argue the point with some small-minded official, better not to attract attention in the first place.

Two of the bedroom walls in our poptop are built to fold outwards to create, you guessed it, more shutters. This does three things, firstly it provides more shade for the sides and roof of the motor home. Secondly it allows the bedroom to be almost entirely open (with insect screens of course) to catch any passing breeze, and thirdly it provides egress for hot air at the very top of the motor home which improves the ventilation.

When the pop top is down we have enough headroom to sleep comfortably, and when it's up, we can have full standing room which obviously makes life easier.

In my opinion there is only one choice, a DC compressor fridge (since writing this the Eutectic fridges have come on the market, these may be the best way to go), let me bullet point my reasons.

• DC fridge manufacturers know that the fridge will be used in an application that needs efficiency. Therefore the compressor design is more efficient and the amount of insulation is vastly improved over the domestic model.
• Gas fridges use a non-renewable resource, ie gas. Even though there are supposedly three way (gas/12v/240v) it is not practical to use them with your batteries because the 12v current draw is far too high.
• AC fridges have to be driven via the inverter so as well as being less intrinsically efficient you loose another 10% or so converting the battery's DC power to AC.
• Typically a AC fridge will use 2-3 times the power of its DC equivalent. Even a DC fridge will be your largest consumer of power, but the AC version is diabolical.

Unless your hobby is aluminium smelting your fridge will be the largest power user in the motor home so it makes sense to choose wisely here. Why spend ages selecting the most efficient 10w reading lights when a bad choice of fridge can cost you 1000w per day. I know the DC fridges are not cheap but then this article is about making an energy efficient motor home, not a cheap one.

So why are 240v fridges less efficient than 12/24v fridges? New energy ratings notwithstanding I guess that manufacturers generally don't worry as much about efficiency with the normal domestic fridge because there's plenty of power available in the home. As I understand it there are three main reasons that an AC fridge is less efficient,

1. The motor that drives the compressor is often a lower quality unit.
2. Domestic units defrost by heating the inside to rapidly remove ice. Heating the inside of a fridge is pretty ridiculous but it is done in the name of convenience.
3. Many AC fridges have less insulation than DC versions.

Another very important factor in fridge energy consumption is the usage pattern. Most people are comfortable with the idea that hot air rises but many have a problem with cold air falling. My wife still thinks I'm joking when I tell her that opening the fridge door will let the cold air fall out of the fridge.

Naturally a family with four kids constantly getting cold drinks and leaving the door open while they fill a glass will use a lot more power than someone who opens the fridge twice a day. This is the reason that it is more efficient to keep the fridge as full as possible so there is less air to fall out. As long as the contents don't fall out every time you open the door, the cold will be retained so the fridge won't have to work as hard when you close the door.

For this reason chest-type fridges are more efficient because the air stays in the fridge when the door is open. However most people would prefer a normal upright style in a kitchen so how about trying to stop the majority of this air loss by Installing some see through doors on the shelves or using bins similar to the salad crisper for all the food. This will limit the amount of cold air that is lost when the door is opened.

So how much power does a fridge use? Bearing in mind the above about usage patterns, here are some figures from a table recently published in ReNew magazine. I'll just pick some indicative units of around 200 litre capacity.

Note that these figures seem to have been obtained at different ambient temperatures which makes them hard to compare. Use them as a rough guide only. Also, it's probably safe to assume that the manufacturer's tests were not done with kids opening the door every five minutes so you'll have to add a fudge factor based on your own usage.

We bought a converted Fisher & Paykel even though the above figures may indicate it's not the most efficient. We bought it because it was the right size (240ltr), had a large freezer (almost half the fridge's volume), it's an upside down model with the freezer at the bottom and we have a very large solar power system. So you see there are other things to consider as well as the energy efficiency.

Gas or electric
OK I just couldn't leave this debate alone. Why would you choose gas over electric or vice versa? The following table shows some of the pros and cons for each type.

At every rally I attend the conversation invariably gets around to keeping gas fridges cold. Not once have I ever heard a compressor fridge owner discussing tricks for leveling this or mounting thermometers on that. This tells me that, if you want the convenience of an appliance that you just set and forget then compressor fridges are the way to go. But they do use a lot of electricity, even the good ones, so you need two or three more solar panels to compensate.

If I were pushed I would recommend the following,

In a small rig or camper van that can easily be leveled and with limited room for a solar power system, use a gas fridge.

In a large rig that cannot easily be leveled (try putting blocks under a bogie drive) and that can host a large power system, use a compressor fridge.